1. Field of the Invention
The present invention relates to a power control apparatus. More particularly, the present invention relates to an apparatus for automatically adjusting an input signal to a desired power level and outputting the obtained result and to an apparatus suitable for, e.g., a radio transmitter adopting TDMA (Time Division Multiple Access) for carrying out burst transmission.
2. Description of the Background Art
FIG. 19 is a block diagram showing a structure of a conventional power control apparatus, and such an apparatus is disclosed in, for example, Japanese patent laid-open publication No. Hei4-354209. In FIG. 19, the power control apparatus is equipped with: a power amplifier 102; a directional coupler 103; a transmitting antenna 104; a cymoscope 105: direct-current amplifiers 106 and 112; A/D converters 107 and 115; a CPU 108; a PROM 109; a pulse width modulation generator 110; a low-pass filter 111; a temperature detector 113; and a direct-current converter 114.
A modulated high-frequency signal is amplified by the power amplifier 102 and divided into power to be supplied to the transmitting antenna 104 and that to be supplied to the cymoscope 105 by the directional coupler 103. The power supplied to the cymoscope 105 serves as control power for controlling the electric power to be supplied to the transmitting antenna 104, i.e., an output level of a carrier wave.
The control power is detected by the cymoscope 105 and converted into a direct voltage to be amplified by the direct-current amplifier 106. The direct-current voltage amplified by the direct-current amplifier 106 is digital-coded by the A/D converter 107 and fetched in the CPU 108.
Meanwhile, a circuit temperature of the power control apparatus is detected by the temperature detector 113 and supplied to the direct-current converter 114 as a temperature signal. The temperature signal is converted into a direct-current voltage by the direct-current converter 114 and then digitized by the A/D converter 105 to be fetched in the CPU 108.
The CPU 108 generates an appropriate correction instruction signal associated with the circuit temperature based on the above-mentioned two types of digital input data and correction data stored in an PROM 109. The correction instruction signal is fed to the PWM generator 110 in which a pulse-width-modulated control voltage is generated. This control voltage is converted into a direct current voltage by the low-pass filter 111 and amplified by the direct-current amplifier 118. The control voltage amplified.by the direct-current amplifier 118 is supplied to the power amplifier 102 in order to control an output from the power amplifier 102.
In such a conventional power control apparatus, however, a time constant of the low-pass filter 111 for converting the control voltage into the direct-current voltage is a fixed value determined in accordance with characteristics of the lower-pass filter. The control voltage may discretely vary by an amount or period of renewal of the control voltage and, in this case, the low-pass filter having a fixed time constant has such a problem as that the discrete change in the control voltage can not be sufficiently smoothed to stabilize the control loop.
Further, when a low-pass filter having a large time constant is used as the low-pass filter 111 in order to solve the above-described problem, in the high-speed burst transmission of a radio communication apparatus adopting TDMA for example, the rising of the transmitted power slows down to collapse data in the burst leading edge.
Furthermore, if a change in the ambient temperature or a power supply voltage fluctuation of a circuit is observed, the overshoot or undershoot of the transmitted power is generated immediately after start of transmission and immediately after stop of transmission due to the environmental capability, the bias change and the like of the direct-current amplifier, the power amplifier and others.